1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus.
2. Description of Related Art
In electrophotographic image forming apparatuses, image densities, line widths and print positions change according to environmental or endurance changes of temperature, humidity and such like due to the characteristic of image formation using static electricity, and it has been difficult to output images stably.
Thus, in recent years, there have been apparatuses that can correct the changes including changes at secondary transferring sections and fixing sections by detecting color patches on sheets with line sensors set in sheet conveyance paths after fixation and feeding back the detection results to image forming conditions (see Japanese Patent Application Laid-Open Publication Nos. 2006-71693 and 2005-39364). This control ensures stable quality in each individual image forming apparatus.
However, in addition to quality assurance in individual image forming apparatuses, there has been a growing need for assurance of color absolute values especially in color image forming apparatuses.
Thus, there have been developed apparatuses which calibrate line sensors on the basis of results obtained by measuring color patches on sheets with spectrophotometers. That is, it is possible to estimate values equivalent to reading information (color values such as XYZ values and L*a*b* values) of spectrophotometers from reading information (RGB values and such like) of line sensors.
In image reading devices such as line sensors and spectrophotometers, backing members influence measurement values. The backing members are members set as background of sheets when images are read out. For example, a sample is placed on several layered sheets (white paper) which are the same paper as the sample to be measured and an image is read out by the image reading device.
The backing members are classified roughly into white backing members and black backing members. For example, when a white holder and a black holder are prepared as a copyholder of a flat head scanner, the user can select and purchase one of them. When a thin sheet is scanned by using a black backing member, the density is detected as higher at a background portion and a low density portion of the scanned image in some cases. The white backing members have various characteristics and are specified in detail in ISO 13655:2009, for example, to be opaque, to have a diffusing surface, not to include a fluorescent material, and to have a certain range of color saturation and luminosity as white backing conditions.
Accordingly, there has been possibility that accuracy of estimation calculation is lowered when two image reading devices use different backing members in a case where reading information of one image reading device is estimated from reading information of the other image reading device as in the above conventional techniques.